Abstract Worry is a habitual thought process that is observed in individuals with anxiety disorders. A leading theory posits that although worrying may be unpleasant, the immediate emotions that are avoided by concentrating on worry are often perceived as more aversive (eg, fear, anger, grief). From a mechanistic perspective, worry is thought to be learned and reinforced in a similar manner to other types of operantly conditioned behaviors. Therefore, with cycles of reinforcement learning, the individual with an anxiety disorder learns a maladaptive, distracting thinking style that uses worry to focus on the future rather than the present. Worry is a major obstacle to sleep, through increasing physiological arousal. Thus, there is an important need to develop treatments that decrease worry thinking, as they may have significant impact on both anxiety and sleep. Mindfulness training involves focusing on cognitive, affective, and physiological experiences occurring in the present moment. Mindfulness teaches individuals to focus on current thoughts, feelings, and physiological sensations, no matter what the present experience might entail. As such, one may learn to consciously act rather than unconsciously react to external or internal stimuli. With mindfulness practice, individuals learn to move away from maladaptive habitual thought processes, such as worry as a distraction strategy. We have developed a digital therapeutic program for anxiety that delivers mindfulness training via an app-based platform to help adults overcome anxiety (Unwinding Anxiety). We have designed it to mechanistically target the reinforcement learning process. We propose to test engagement of specific mechanistic targets of reinforcement learning in individuals who report worry that interferes with their sleep. Individuals will be randomized to either Unwinding Anxiety or waitlist control. Our aims are to 1) Determine the degree to which mindfulness training affects maladaptive reinforcement learning, and 2) Test the degree to which mindfulness training reduces worry-driven sleep disturbance. This project will be the first to study the ability of a mobile mindfulness training to engage worry and the resultant effects on sleep. The knowledge gained will set the stage for a number of future studies related to further elucidating the mechanisms of mindfulness, and the clinical efficacy and utility of this type of training delivered via digital therapeutics in general. Specifically, this study will provide critical information on which science of behavior change assays are most robust for measuring target engagement, and will provide necessary variances for powering larger studies.